The present invention relates to improvements in catalytic reactors of a kind comprising a metal substrate body. Generally, such a substrate body consists of flat thin metal (foil) strips alternating with corrugated thin metal (foil) strips. The thickness of the flat and thin foil strips is typically in the range of 0.05 to 0.1 mm. The strips are wound upon themselves about an axis so as to form passages extending axially through the substrate body for through-flow and catalytic purification of exhaust gases.
To achieve the desired catalytic purification, a coating (so called washcoat) is applied on the foil strips, said coating usually consisting of an aluminum oxide and noble metals (such as e.g. rhodium, platinum, palladium).
The thus-wound substrate body is provided with an enclosing metal mantle. According to prior-art technology, this mantle has a sheet thickness of between 1 and 1.5 mm. The reason for the relatively large thickness of the mantle is to make it possible to fasten the mantle, by welding or brazing, to the substrate body and to a casing (canning) surrounding the mantle.
One problem encountered in a thus-structured catalytic reactor is its deficient strength. When hot gases (up to 1000.degree. C.) flow through the passages, the foil strips are rapidly heated, and as a result the substrate body expands axially and radially. The surrounding mantle, on the other hand, is not directly exposed to the gas flow. Since in addition thereto the mantle is much thicker and thus has a larger mass to be heated than the foil strips, it will expand at a much lower rate. In consequence thereof, a considerable compression force will be generated in the space between the mantle and the outermost, thin layer of the substrate body. That is a potential cause of deformation of the outer corrugations in the substrate body with consequential destruction of the passages in that layer.
When the substrate body is cooling, the opposite problem arises. The foil strips are cooled at a much higher rate than is the considerably thicker mantle, the latter, as already mentioned, having no direct contact with the gas flow. Consequently, the substrate body will contract much quicker than the mantle. If, in this situation, the individual foil strips are joined together, considerable tension will be generated between the layers in the radial direction as a result of the differences in the extent of contraction between the substrate body and the mantle.
In substrate bodies wherein the coating is the only bonding agent, the strength of the bond will be exceeded and cracks and gaps form, usually in a couple of layers closest to the mantle. In substrate bodies having a diameter size of about 100 mm, a gap of 1 mm may form.
Considerable tension is generated in substrate bodies of the kind wherein the layers are joined together by brazing. Particularly the tension between the outermost layers of the substrate body and the mantle will be of such a magnitude that the brazed bonds run the risk of disrupting. The strength of the catalytic reactor is seriously affected by these problems.
The difference in expansion between the mantle and the substrate body is the principal reason for the generation of compression or tensile stress. The object of the invention is to provide a catalytic reactor having a mantle which is more adaptable to the motions of the substrate body and thus is able to prevent the substrate body from being exposed to harmful mechanical compression or tensile stress.